The invention relates to Code Division Multiple Access communication systems and, more particularly, to methods and apparatus for providing downlink diversity in such systems employing Walsh codes.
Consider a Code Division Multiple Access (CDMA) system that uses orthogonal Walsh functions to separate users on the downlink along with a base station specific covering code. It is well known that xe2x80x9cdownlinkxe2x80x9d refers to the communication path or paths from a base station to a mobile terminal or station (hereinafter referred to as a xe2x80x9cmobilexe2x80x9d), as compared to the xe2x80x9cuplinkxe2x80x9d which is the communication path or paths from a mobile to a base station. For a system with K mobiles receiving signals from a common base station, the transmitted signal on a single antenna may be represented as:                               x          ⁡                      (            t            )                          =                              (                                                            ∑                                      i                    =                    1                                    K                                ⁢                                  xe2x80x83                                ⁢                                                                            P                      i                                                        ⁢                                                            s                      i                                        ⁡                                          (                      t                      )                                                        ⁢                                                            w                      i                                        ⁡                                          (                      t                      )                                                                                  +                                                                    P                    p                                                  ⁢                                                      w                    0                                    ⁡                                      (                    t                    )                                                                        )                    ⁢                      p            ⁡                          (              t              )                                                          (        1        )            
where Pi is the power transmitted to the ith mobile, si(t) and wi(t) are the data signal and unique Walsh function intended for the ith mobile, respectively, Pp is the power of the pilot signal which uses Walsh function 0, and p(t) is the covering code for the base station of interest. Further, the Walsh functions are orthogonal and repeat every symbol time Ts, i.e.:                                           ∫            0                          T              s                                ⁢                                                    w                i                            ⁡                              (                t                )                                      ⁢                                          w                j                            ⁢                              xe2x80x83                            (              t              )                        ⁢                          ⅆ              t                                      =                  {                                                    1                                                              i                  =                  j                                                                                    0                                                              i                  ≠                  j                                                                                        (        2        )            
At the mobile, the following signal is received on a single antenna:
xe2x80x83y(t)=h(t)x(t)+n(t)xe2x80x83xe2x80x83(3)
where h(t) is the complex multiplicative distortion caused by the wireless channel and n(t) is thermal noise. Mobile i correlates the received signal with the ith Walsh function during the kth symbol interval after uncovering to achieve the decision statistic zl[k]:                                                                                           z                  i                                ⁡                                  [                  k                  ]                                            =                                                ∫                                                            (                                              k                        -                        1                                            )                                        ⁢                                          T                      s                                                                            kT                    s                                                  ⁢                                                      y                    ⁡                                          (                      t                      )                                                        ⁢                                                            p                      *                                        ⁡                                          (                      t                      )                                                        ⁢                                                            w                      i                                        ⁡                                          (                      t                      )                                                        ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    t                                                                                                                          =                                                                                          P                      i                                                        ⁢                                      h                    ⁡                                          [                      k                      ]                                                        ⁢                                                            s                      i                                        ⁡                                          [                      k                      ]                                                                      +                                  n                  ⁡                                      [                    k                    ]                                                                                                                    (          4          )                ⁢                  xe2x80x83                    
where h[k] represents the cumulative effect of the channel h(t) over the kth symbol interval, and si[k] is the kth transmitted symbol for the ith mobile.
The transmitted symbol can be recovered by using an estimate of the channel, ĥ[k], obtainable from the pilot channel, i.e.:
ŝi[k]=f(zi[k]ĥ*[k])xe2x80x83xe2x80x83(5)
where f(xc2x7) is an appropriate decision function. Assuming a flat, Rayleigh faded channel, in the absence of fast, accurate power control, the resulting performance of the link will be rather poor due to the lack of diversity. As a result, it is desirable to have a second antenna at the receiver to allow diversity reception, improving performance considerably. However, mobile handsets do not easily allow a second antenna to be added. Thus, methods of achieving diversity performance from the transmitter have been proposed.
One method of achieving diversity performance is to transmit the same signals on multiple carriers. However, this is wasteful of the one resource that cannot afford to be wasted in mobile communications, namely, bandwidth. A second more reasonable alternative is delay diversity. This method purposely causes multipath by transmitting the signal twice from the base station with the second transmission delayed in time by several chips and occurring on a separate antenna. By dividing power over two transmissions transmit power is not increased, but time diversity is provided which can easily be exploited by the mobile""s Rake receiver with no required changes. However, the scheme is ultimately limited by the self-interference caused by this intentional multipath. Since all user signals are transmitted synchronously, this multipath interference can be quite large, especially for a moderate to heavily loaded system.
Other techniques have been proposed for attempting to improve diversity in the area of space-time coding. For example, a simple two-branch transmission diversity technique is described in Siavash M. Alamouti, xe2x80x9cA Simple Transmit Diversity Technique for Wireless Communications,xe2x80x9d IEEE Journal On Select Areas In Communications, Vol. 16, No. 8, October 1998, the disclosure of which is incorporated herein by reference. Further, the use of channel codes for improving the data rate and the reliability of communications over fading channels using multiple transmit antennas is described in Vahid Tarokh et al., xe2x80x9cSpace-Time Codes for High Data Rate Wireless Communication: Performance Criterion and Code Construction,xe2x80x9d IEEE Transactions On Information Theory, Vol. 44, No. 2, March 1998, the disclosure of which is incorporated herein by reference. Still further, a technique for coding messages for transmission on the downlink so as to use multiple transmitting antennas for improved reception in fading environments is described in U.S. provisional application Serial No. 60/114,621, filed on Jan. 4, 1999, entitled: xe2x80x9cSpace-Time Spreading Method of CDMA Wireless Communication,xe2x80x9d which is the basis for a U.S. non-provisional application filed on Apr. 2, 1999 having the same title.
Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made by one skilled in the art without departing from the scope or spirit of the invention.
In any case, each transmit antenna transmits a signal representing the result of the modulation of Walsh codes by data signals for each of the K mobiles, assuming there is data pertaining to a particular mobile. For example, a first transmit antenna may transmit a signal that may include: a component that is the result of a first Walsh code modulated by a data signal associated with a first mobile; a component that is the result of a second Walsh code modulated by a data signal associated with a second mobile; up to and including a component that is the result of a Kth Walsh code modulated by a data signal associated with a Kth mobile. Then, a second transmit antenna may transmit a signal that may include: a component that is the result of the Kth Walsh code modulated by a data signal associated with a first mobile; a component that is the result of the (Kxe2x88x921)th Walsh code modulated by a data signal associated with a second mobile; up to and including a component that is the result of the first Walsh code modulated by a data signal associated with a Kth mobile. As is evident, in this example, the same number of Walsh codes are used in conjunction with the second antenna as are used at the first antenna, but simply redistributed with respect to the data signals that respectively modulate them. The ordinals assignment of users may be arbitrary, that is, any user can be specified as user K. However, the relative assignment of Walsh codes is accomplished in accordance with a transmission matrix T. The transmission matrix maps the user data symbols onto the Walsh codes for each antenna and is preferably designed such that its columns are representative of the transmit antennas and are orthogonal. The rows of the matrix are representative of orthogonal channels such as the Walsh codes and frequency bands. Similar redistribution occurs for each of the M transmit antennas such that the respective component in each of the M transmit signals associated with a given mobile""s data signal modulates a unique Walsh code. Thus, at a given mobile, the mobile need only correlate with M different Walsh codes since each transmit signal received will contain a component that is the result of M different Walsh codes having been modulated by a data signal associated with that particular mobile. However, it should be noted that in specific embodiments, less than M different Walsh codes may be used. It is also to be appreciated that the M antennas at the base station are preferably independent or uncorrelated with respect to one another.
As will be explained below, the set of Walsh codes to select from includes 2┌log2(K)┐ Walsh codes. However, with respect to any given mobile, there is needed only as many different Walsh codes as there are transmit antennas serving the particular mobile. The results of the respective M correlations are combined and, with obtained channel estimates, the receiver estimates the particular data pertaining to that particular mobile so that it can be provided to the user. It is to be appreciated that K can be less than the total number of mobiles with which the base station is capable of establishing communications. Also, the present invention provides for various ways to estimate the respective channels between the M transmit antennas and a mobile. Accordingly, the present invention provides improved downlink diversity using Walsh codes in such a manner that: (i) does not require additional receive antennas at a mobile; (ii) does not waste bandwidth; and (iii) does not cause self-interference.
It is to be appreciated that the downlink diversity techniques of the invention may be applied to the physical layer proposed in the IS-95C physical layer draft proposal. This form of diversity advantageously extends the orthogonal transmit diversity (OTD) option of the draft proposal, which obtains only minimal coding gain, and provides true diversity gain. The present invention provides various IS-95C direct-spread and multi-carrier embodiments. However, it is to be understood that the techniques of the invention are more widely applicable to any CDMA communication systems or the like.